I've heard of wooden mortars being used in a pinch before, but this is the first time I've read about mortars fashioned out of rock, or more precisely, carved in natural rock formations. These "stone mortars" were cut into the rock by the Knight Hospitallers of St. John (Maltese Knights), and discovered by Napoleon's troops when the French invaded Malta in 1798.
http://napoleonic-literature.com/Book_20/Lawson-Manuscript.htm#4
"ROCK MORTARS. 23"
"THESE are excavations resembling the interior of mortars, formed out of solid rock, of which there are several in the Island of Malta, executed upon a large scale for the defence of harbours, &c.
Having heard much of this species of Artillery, it excited a curiosity (in our return from Egypt) to experiment two of the most perfect, which are situated in St. Julian’s and St. George’s Bays. The drawing (Plate VIII.) represents a section of one as loaded and fired on the 24th February, 1802.
1st. At St. Julian’s—140 pounds of powder was enclosed in a sort of cask prepared to fit the chamber of the mortar, and being lodged there, a large cane tube filled with quickmatch, was applied to a groove cut along the upper surface of the bore to receive it, and a bottom of wood covered the chamber. The stones were then piled, by four men, within the mortar as they were brought to it in baskets containing about 120 lbs. each: a dozen stones weighing from 120 to 80 lbs. each were first put in, then fifty baskets of other stones from 60 to 30 lbs., then fifty more of 20 to 5 lbs. each, amounting in the whole to upwards of 10 tons. This operation being completed, a piece of portfire was fixed to the end of the tube at the mouth, which communicated through a hole in the cask to the powder. In this manner it was fired as is usual in proving ordnance, and ranged about 700 yards.
2nd. The same mortar was loaded with 180 pounds of powder, and about 10 tons of stones; they spread considerably more than the first time, but did not range quite so far. The explosion this time cracked the mortar in a direction nearly vertical, leaving a fissure in the rock about one-twelfth of an inch wide, ten feet in the rear and four feet in front.
3rd. The other mortar of similar dimensions was fired with a charge rather larger than the first of these, but the effect was not quite so considerable: from whence it is concluded that the first proportion should not be exceeded, especially if after repeated. The stones used in these experiments were chiefly fragments of the rock, which having been exposed to the air were become something harder, and did not suffer so much as might be expected from such violent explosions, which in some degree resembled the tremendous discharge of a volcano."
